MCC and OCR-to-CPTU method for strength assessment in Hiep Thanh offshore wind power project

Piezocone penetration test (CPTU) is one of the most widely used method for geotechnical investigation in offshore power projects. As a case study, direct quick shear, triaxial UU and triaxial CU tests and CPTU were carried out in Hiep Thanh offshore wind power project in south Vietnam to analyse the strength of clayey soil. Based on the laboratory test results, the undrained shear strength .s
u
is found to be linear dependent on the soil depth both for top organic clay (OH/OL) and bottom clay (CH/CL). Negative linear relation had been found between liquidity index I
L
and (s
u
/σ
v0
’)
DSq
. Based on Modified Cam-Clay (MCC) model and laboratory test results, the normalised undrained shear strength (s
u
/σ
v0
')
NC
of normally consolidated organic clay at Om to 18.5m and clay from 20m to 80m are calculated as 0.16 and 0.21, respectively. OCR-to-CPTU interpretation by using the calculated (s
u
/ σ
v0
')
NC
value from MCC model shows a good agreement with laboratory test results of the organic clay at 0m to 18.5m, which preliminarily verifies the validity of this method. However, due to the increasing sample disturbance with soil depth, the interpreted undrained shear strengths are higher than the laboratory test results below 30m. CPTU interpretation are more reliable to represent the mechanical properties of clay layers below 30m. MCC and OCR-to-CPTU method could be used for undrained shear strength interpretation of other offshore wind power projects in the future.